This application claims the priority of German patent document 100 11 562.4, filed Mar. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a gas sensor having functional layer and an electrode structure mounted on a substrate, with an electric heater.
More exacting requirements for environmental protection and air quality demand both engineering solutions to improve the air quality, and measures for monitoring the air quality. For cost reasons it is desirable to avoid the use of expensive gas analysis equipment, and, instead, to use small gas sensors, which are inexpensive to produce, as the detectors for the air quality. One application, where the most exacting requirements for long service life and immunity to interference under the rawest surrounding atmospheres are demanded, is the exhaust gas of an automobile. In this respect very special sensors, which select specific gases, are required for varying drive concepts.
Such gas sensors, which are usually operated at temperatures in the range of several hundred degrees Celsius, can be produced inexpensively by means of planar technology and generally exhibit a layout as described in FIG. 1. A heater and/or a temperature measuring device 4 in the form of a resistance thermometer is/are applied on the sensor bottom side of a transducer, which generally includes an electrically insulating substrate 1. The heater and/or temperature measuring device has leads, which are supposed to exhibit minimum lead resistance (labeled Rlead in FIG. 4), and comprise(s) a heating and temperature measuring structure (Rheating in FIG. 4) that frequently has a meandering shape. An electrode structure 6, which is adapted to the special requirements, is then applied on the upper side of the sensor. On the electrode structure is applied a functional layer 7, which determines the special properties of the sensor, such as the selectivity for a specific gas, etc. This functional layer changes its electrical properties as a function of the composition of the gas atmosphere surrounding the sensor. At the sensor tip a constant temperature is supposed to prevail on the sensor upper side in the area where the functional layer is applied. The constant temperature is adjusted to a specific temperature, the so-called working temperature, by means of the heating and temperature probe on the sensor bottom side.
The ambient atmosphere-dependent electric properties of the functional layer are called the measured variable in the following. For example, it can be: the complex impedance Z or derived variables, such as the capacitance, the loss resistance, the phase angle or the magnitude of the complex impedance. In the case of a measuring frequency of O Hz (d.c. voltage) the direct current resistance must also be defined as the measured variable. In the case of a highly resistive functional layer, an interdigitated capacitor structure (IDC) is used as the electrode structure, as sketched in Plog C., Maunz W., Kurzweil P., Obermeier E., Scheibe C.: Combustion Gas Sensitivity of Zeolite Layers on Thin Film Capacitors. xe2x80x9cSensors and Actuatorsxe2x80x9d, B 24-25, (1995), 403-406 and in German patent documents DE 19703796, EP 0527259 or DE 19635977. In the case of highly conductive samples, an electrode arrangement with large electrode intervals is best provided, such as in the German patent document DE 19744316, where a four conductor arrangement was selected as the electrode arrangement for a semi-conducting titanate as the functional layer.
However, the measured variable can also be an electromotive force (EMF) between two electrodes, e.g., of an ionic conductor. Thermoelectric voltages can also be a measured variable. In the case of a limiting current sensor the current that flows when a voltage U is applied, is a function of the measuring gas concentration and thus the measured variable.
Typical gas sensors, which are constructed according to the above pattern, can be derived from the following documents. European patent document EP 0426989 presents a selective HC sensor, whose capacitance changes with the gas test. German patent document DE 19703796 discloses a selective ammonia sensor, whose loss resistance and capacitance change as a function of the gas concentration in the range of 20 Hz to 1 MHz. German patent documents DE 19756891, DE 19744316, EP 0498916 and DE 4324659 disclose titanate-based oxygen sensors, whose d.c. resistance at several hundred degrees Celsius depends on the oxygen partial pressure of the surrounding gas. Even German patent document DE-3723051 describes such sensors.
An HC sensor, which comprises two resistive oxygen sensors and is also produced using planar technology, is disclosed in German patent document DE 4228052. A typical, planar limiting current sensor for measuring the oxygen content and other components of a gas, is described in Gxc3x6tz R., Rohlf L., Mayer R., Rxc3x6sch M., Gxc3x6pel W.: Amperometric Multielectrode Sensor for NOx and Hydrocarbons: Numerical Optimization of operation Parameters and Cell Geometries. Proceedings to xe2x80x9csensor 99xe2x80x9d, May 18-20, 1999, Nurnberg, pp. 137-142. A gas sensor which utilizes thermoelectric voltage as the measuring effect, and the necessary electrode arrangement, are described in German patent document DE 19853595.
Since such sensors are operated at several hundred degrees Celsius, they must be heated. However, the insulating properties of the substrates that are used are no longer optimal at these temperatures and as a result, the measured variable is affected by the coupling of the voltages required to heat the sensor.
This disadvantage will be explained in the following three examples.
The sensor is operated at a d.c. voltage U0. In this case a potential distribution, as depicted in FIG. 2, is applied to the sensor heating arrangement 4 on the sensor bottom side. U0=10 V was chosen as an example in FIG. 2. Owing to the electrostatic field distribution, a voltage, which can be measured as the voltage Uk between the electrodes on the sensor upper side, is induced on the sensor upper side. In an equivalent electric circuit one must imagine an ideal voltage source that delivers the voltage Uk, with a resistor (the internal resistance of the voltage source) connected in series. Said resistance depends on the dielectric constant and the d.c. insulation resistance of the substrate. Of course, the amount of the voltage Uk also depends on the electrode arrangement (electrode spacing, electrode width, direction of the electrode, etc.), the arrangement of the heating resistors and the substrate thickness. Correspondingly a voltage Uk, which renders the measurement of the measured variable difficult, is measured over the electrodes. This voltage is defined as the bias voltage, which can also lead to a change in the functional layer and thus to a falsification of the measurement signal. When this voltage is applied for a prolonged period of time, it can also result in a drift in the measured variable.
The sensor is operated on alternating voltage with the amplitude U0 at the frequency f0. In this case the description under example 1 is also applicable. The situation is complicated by the fact that the electric insulation resistance of a typical substrate decreases with the frequency. For a commercially available Al2O3 substrate for thick film technology (96% purity, specific d.c. volume resistance $ greater than 1010 3 m) the behavior of the specific, i.e. volume resistance $, corrected for geometric influences, is plotted as a function of the temperature T and the measuring frequency f in FIG. 3. It is easy to recognize that precisely in the range of high temperatures and high frequencies the insulation resistance decreases drastically. In such a case the alternating voltage of the heating arrangement is coupled through the substrate, and is in the same phase over the measuring electrodes.
In the case of a sensor that measures the EMF between two electrodes, the coupled alternating voltage will overlay the EMF and will falsify the measured variable. For this case FIG. 4 depicts an equivalent electric circuit. As a function of the dielectric constant and the thickness of the substrate, resulting from the substrate capacitance, labeled as Csubstrate, there is, in addition to the coupling denoted by the ohmic loss resistance Rsubstrate, also a capacitive coupling, which is out of phase by 90 degrees and can be measured at the electrodes. The capacitive coupling takes place over the capacitive voltage divider Csubstratexe2x88x92Cinternal, where Cinternal reflects the capacitive resistance and Rinternal reflects the internal resistance of the sensor.
In the case of highly resistive functional layers with low capacitance, where the minimum capacitance change must be measured in the pF (=pico-farad) range, as presented in the DE 19635977 or the EP 0426989, the coupled alternating voltage can have a significant effect on the finding of the measured variable.
The sensor is operated by means of a constant, but pulsed direct voltage U0 at variable pulse width. Since it is the most energy efficient, it is probably the most common operating mode for a gas sensor, because only the two voltage states U0 and approx. 0 V are applied to the sensor heater, and there is virtually no loss of power over a series resistance. In this case a combination of the effects, described in the examples 1 and 2, occurs. At the instant that the heating process starts, U0 is applied; and the description under example 1 applies. Due to the steep edges, during the turn on and turn off process there are many frequencies in the spectrum. These frequencies are perceived on the sensor upper side in the form of interferences and make it more complicated to determine the measured variable, falsify the findings, or even render it impossible to determine the measured variable.
The object of the invention is to overcome the aforementioned drawbacks of the known gas sensors.
This and other objects and advantages are achieved by the gas sensor according to the invention, in which an electrically, highly conductive (in particular, metallic) shielding structure is disposed between the heating arrangement and the sensor unit, the latter comprising a functional layer and an electrode structure. The shielding structure can comprise, for example, a closed layer. Similarly a network-shaped design or a structure in the form of a line pattern, for example, several parallel conducting tracks, is possible.
A suitable material for the shielding structure is a precious metal, such as platinum (Pt) or gold (Au) or a Pd/Ag alloy, while ceramic materials, such as Al2O3, MgO or AlN, can be used as the substrate materials. The functional layer of the sensor unit can be made, e.g., of zirconium oxide, a titanate, a zeolite or xcex2xe2x80x3xe2x80x94Al2O3.
The sensor unit can be designed such that the complex impedance of the functional layer or the derived variables serve as the measured variable. In addition, the measured variable of the sensor can be an electromotive force, a thermoelectric voltage or an electric current.